Stephen Hoover wrote: > > Ken, > > Thanks for the input on this discussion. I follow and > understand your > example without any problems. > > Now if taking it back to the original original question - > Does L3 > switching require VLANs - produces this question for your > example: > > You state 1 fiber feed for both Science and Engineering in > the Labs > building. I am then assuming that they are all connected to the > same set of > switches (Layer 2) in that building. > Could you have not just simply assigned the hosts for > Science to 1 IP > network and the hosts for Engineering to another IP network - > then created > respective gateway interfaces for each network back on the > common Layer 3 > switch and accomplished the same thing??
It depends on the meaning of "thing" in your "accomplish the same thing" comment. :-) I think you already figured out your confusion and maybe this message is old, but I'll reply just in case. With your design you would accomplish connectivity. However, you would not accomplish separation of broadcast traffic for the two user communities. VLANs in the L2-switched part of the network give you that. VLANs have lots of features, but that's one of their primary ones. I think Ken's example is one of the cleanest I've seen. I may have to borrow it for my classes. Thanks for a good discussion, Stephen. THE END (hopefully! :-) Priscilla > > If the answer is yes, I will followup with another > question. If the > answer is no, then please explain. > > Thanks!! > > Stephen > ----- Original Message ----- > From: "Ken Diliberto" > To: > Sent: Tuesday, February 18, 2003 12:24 AM > Subject: Re: Does MLS (Layer 3 switching) require VLANs? > [7:63147] > > > > Stephen, > > > > You're getting there. Let me give an example of how VLANs > are used > > (I'd draw a picture, but it probably wouldn't look good). > > > > For this example, let's use two of the colleges on my > university > > network: Science and Engineering. > > > > Each has their own block of IP addresses and want their > traffic > > separate from the other. They also want flat addressing (no > > subnetting). > > > > We have three buildings: Science, Engineering and Labs. > Science and > > Engineering both have computer labs in the Labs building. > Each want > > their labs on their respective IP address blocks. > > > > If money were no object, this would be fairly easy with > vanilla > > switches and a router with two ethernet interfaces. Multiple > fiber > > feeds and two sets of switches would be everywhere. > > > > With budget limitations (for this example), we only have a > single fiber > > feed to each location. That means each fiber feed needs to > carry > > traffic for both networks. To keep the traffic separate, we > partition > > the switch ports into two LANs: LAN 10 and LAN 20. These two > LANs in > > one switch are treated as unique. To do this, the switch > creates > > Virtual LANs or VLANs. The fiber feeds are now trunks > because a header > > is added to each frame to identify the VLAN it belongs to. > > > > So far so good? > > > > Why would we need a router? To talk between VLANs. > > > > Do routers understand trunks? Yes. > > > > This brings up one more concept: the Router on a Stick. > > > > A router on a stick is a router with a single network > connection. This > > single connection is configured as a trunk so the router can > see all the > > different VLANs. If the router finds a packet on VLAN 10 > with a > > destination on VLAN 20, it rewrites the headers for the > destination and > > puts it back on the same trunk with VLAN 20 headers. > > > > Remember: replace "layer 3 switch" with "router" every time > you see > > it. That might make more sense. > > > > Hope this helps. > > > > Ken > > > > >>> "Stephen Hoover" 02/17/03 > 06:55PM >>> > > I appreciate everyone's input on this subject to help me > understand > > this > > concept. > > > > As far as the newbies comment goes - I most definitely am. > I'm about > > as > > green as they come. I have both my CCNA and my CCDA, but my > only real > > experience is installing 2 T1s (at different locations) and > configuring > > NAT > > for them. I have large amount of knowledge, just no > experience. It has > > been > > my goal and my dream to become a serious network engineer for > the last > > 6 > > years, but I just cannot seem to get a job that offers any > experience. > > Everytime I get a "network" position, I just seemed to end up > doing > > desktop > > support. > > > > When I first heard the term Layer 3 switching (some 4 years > ago now) > > the > > first thing that popped into my mind was a switch that can > route. I > > never > > even heard of a VLAN until a couple of years ago. > > > > The Cisco Study guide starts off talking about VLANs, and > moves right > > into > > Inter-VLAN routing without ever really discussing Layer 3 > switching as > > a > > seperate process. This is really where my confusion started. > The book > > makes > > it sound like L3 switching is directly dependent on VLANs, > and I just > > didn't > > see it - it wasn't something I was just willing to accept. > > > > Further more, the book states that VLANs allow for physical > location > > independence, but is also says that VLANs should not cross > the core - > > those > > 2 statements seem partly contradictory to me. > > > > Here is a summary of how I see VLANs now. > > > > Layer 3 switching is possible without VLANs (however the > opposite is > > not > > true. Well at least not without some form of Layer 3 > intervention.) > > > > VLANs simply the administration behind Layer 3 switching > design. > > > > Physical location (port location) independence is ok in > front of the > > layer 3 switch that is the the hosts gateway. Up to the hosts > > distribution > > switch. > > > > VLANs extending beyond the distribution layer switch > across the > > core is > > generally not a good idea - possible, but not recommended. > This is the > > "flat > > earth" design that Priscilla mentioned - VLANs that extend > across the > > entire > > internetwork. > > > > Thanks! > > Stephen Hoover > > Dallas, Texas > > > > [snip] > > Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=63291&t=63147 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
